Re: DC magnetisation of transfromers and chokes.



Yves Monmagnon wrote:

"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> a écrit dans le message de news:
4420B97B.A9899D46@xxxxxxxxxxxxxxxxxxxxx
The formula for woking out turns on an output transformer is

N = 22.5 x V x 10,000 / ( Afe x B x F )

where N is turns,
V is the voltage in Vrms across the winding,
10,000 is a constant for all equations,
Afe is the core area in sq. mm,
B is the flux density in Tesla,
F is the frequency.

I try to have the ac Bmax = 1.2 Tesla at 14Hz, so the bass response of
any amp is very good.

However, where you have an OPT for an SE amp there is DC in the core and
one has to limit ac Bmax to 0.6T
approximately to allow for 0.6T of dc magnetization of the core
in a transformer carrying DC through its primary.

The formulas I have give the DC flux in Tesla = 12.6 x µe x N x Idc / (
FeMl x 10,000 ),

where 12.6 is a constant, µe is the effective permeability when a gap is
installed, and is often a value
below 500, and calculated with another formula taking into account the u
max for interleaved material
and the air gap,
N is the primary turns,
Idc is in amps dc,
FeMl = iron core magnetic path length in millimetres,
10,000 is a constant for all equations.

For example, a 100mm stack of 50mm tongue E&I material with 600mA of
Idc, and 1,480 turns and µe = 184
and FeMl = 280mm has a dc field strength

= 12.6 x 184 x 1,480 x 0.6 / ( 280 x 10,000 ) = 0.73 Tesla,
so if total ac and dc flux cannot exceed 1.2T, then
I could have 0.46T max for ac Bmax.


However, if there was not a gap in the transformer, how does one
calculate the DC field strength?
Is µe just µ with ungapped material?
in my case µ is 17,000 maximum for the lams that I have when testing
with ac.

There is a case where I want to build a push-pull amp with two 1,480
turn windings
in series with balanced dc flows in each 1/2 of the transformer, in the
standard fashion.
The PP has the two windings in series, with a CT taken to the B+, and a
fully interleaved stack of
GOSS, 100 mm stack of 50 mm tongue material.

If there is a DC imbalance to the two tubes working in PP, say 6mA, then
I could have a net
dc flow from one end of 2,960 turns to the other of 6mA, so the using
the above formula,

dc Bmax = 12.6 x 17,000 x 2,960 x 0.006 / ( 280 x 10,000 ) = 1.36 Tesla

Mmmh ?
If a tube draw 6mA more current than it's brother (sister ;>) should we
consider this current is flowing in the whole winding or just the half one ?
If so, no more than 0.7 Tesla. Still objectionnable anyway !

The imbalance of dc across the whole PP winding is the imbalance, and equivalent
to a flow
of that much .
Its best to be pessimistic about it and consider that 6mA imblance means +3mA at
one end and
-3mA at the other, rather than +6mA for 1/2 the winding.


This seems a "silly result" because in many amps I have seen a dc
imbalance, or net dc flow in one direction
and seen no saturation problems.

Agreed !


Does anyone have a better idea how to **calculate** the dc flux density
in gapped or ungapped
laminated steel cores?

Not me !


perhaps the µ for dc use varies non linearly with dc flow so that µ is
low at low dc and high with high dc,
similarly to what ac voltage produces.


Seems a good suggestion. Something like a threshold effect ? Dunno !

ac flux density increase with applied voltage is non linear.
At low Vrms, µ is much lower than the maximum, and during each wave cycle,
so that each side of zero V , the instaneous reactance of the inductance is much
lower than
it is at say a couple of hundred Vrms across the OPT.
So hence the iron causes some distortion to the signal due to change in
inductance during each wave cycle.
A pure and constant Lp would not cause any distortion except at LF when ZL
becomes low enogh to be close to RL
which is in shunt with ZLp.


Simply adding DC and AC induction components may be a "sufficient"
approximation as long as they are somewhat similar.
AC flux being large enought to move "magnetics grains" in the steel
regardless the DC component ?

The magnetization of the steel by dc to say 0.8Tesla does not make it much more
difficult for the
ac to "move grains"; the u does reduce a bit with dc before saturation, but ZL
will still be high
compared to RL.
Wiiliamson said that his amp needed at least 100H at a low Va-a of 5Vrms, and in
fact
many Xformers who pass this requirement have less than 100H at less than 5Vrms
a-a,
and hence they sometimes oscillate at LF a little, almost un-noticeably, because
as the voltage tried to increase,
it is limited by an increase in inductance whose value rises and moves to the
value above which oscillation would not occur.
The Williamson had a maximum of 600H when the µ of the GOSS material of that
time reached its maximum
of about 5,000.
I am using E&I GOSS lams which measure 17,000, so Lp is enormous, and I could
get away
with just gluing the Is to all the Es without interleaving them.
Maybe that's a better way to build the tranny....
A toroidal has µ of the same GOSS of maybe 40,000, hence dc imbalances could be
more upsetting.


Another guess: In loaded situation, the variation of the AC current in
primary is large relative the 6mA exemple above so the flux is still
"reversed" at each alternance ? ? ?

True, there is reversal, but with ac voltage, you get 2H distortion.
With 60mA imbalance, not just 6mA, there is a much bigger problem....



Best regards,

Patrick Turner.

OT: A new sheila to spend time with or holydays those last week ;>)

No such luck.

Oz shielas are loud, and expensive, and don't much like bjs.

No holly daze last week either.....

Patrick Turner.



Yves.

.

Relevant Pages

  • Re: Flyback regulator / transformer problems.
    ... I have essentially the same cct with a different transformer, and feedback components changed for a 5Voutput.The other outputs are not regulated, I have only wound the 5V winding and populated the pcb with the required components for 5V out so far. ... The drain voltage of the FET is going WAY to high.Up to about 800V spike. ... The switching frequency is about 100KHz.I am using a EC35 core with N27 material with a 0.5mm gap. ... drive gate with pulse generator at about ...
    (sci.electronics.design)
  • Re: DC magnetisation of transfromers and chokes.
    ... V is the voltage in Vrms across the winding, ... Is µe just µ with ungapped material? ... inductance during each wave cycle. ...
    (rec.audio.tubes)
  • Re: Driving LEDs with a battery pack
    ... that saturate the core. ... the applied voltage hangs around longer and therefore ... winding, but still turns the BJT on. ... collector drops immediately to near zero (there is no collector ...
    (sci.electronics.basics)
  • Re: Flyback regulator / transformer problems.
    ... Tony Williams wrote: ... I have changed my winding scheme to match that mentioned by RL. ... stopped the core from saturating. ... before the voltage falls to 0 it falls at an angle to about ...
    (sci.electronics.design)
  • Re: Calling all transformer gurus
    ... I've removed the high voltage secondary and I'm seeing ... A friend of mine suggested that it might have a shorted winding ... Your transformer is perfectly fine... ... Examine core construction. ...
    (sci.electronics.design)